Remove the built-in chat attachment model in favour of the chat content extension point #507

Goal

The core no longer carries a built-in attachment model. Content attached to chat messages exists only through the generic chat content extension point (ADR 0141), and the spec describes that generic behaviour.

Context

• After the extension content part, sealing and generic blocks are in place, the old path is unused: ChatAttachment, AttachmentKind, ChatAttachmentResolver, validate_attachments, MAX_ATTACHMENT* (crates/core/src/ai/attachments.rs), MessageContent::Attachments, Wiring::chat_attachment_resolvers, ChatAttachmentResolvers (crates/app-shell/src/commands/chat.rs), resolve_chat_attachments, push_user_attachments, redact_attachments (crates/app-logic/src/orchestration/), content_type = 'attachments'.
• Migration 0021 is applied in dev/test databases; sqlx::migrate!() rejects changed or missing migrations, so 0021 and 0022 stay unchanged.
• changelog.d/issue-470-anhaenge-bild-pdf.md describes a change Community users cannot see.

Requirements

• Remove the types and functions listed above and every use; the composer slot, chips, attachment_ids and the ATTACHMENT* codes stay (decided by maintainer).
• Migration 0023: content_type constraint without 'attachments'; rows of that type are deleted (they exist only in unreleased development data). 0021/0022 unchanged.
• Spec 0110 rewritten as generic "extension content in chat messages" (the composer section stays); Spec 0034/0040 attachment wording adjusted; ADR 0137 status "Superseded by 0141".
• Remove changelog.d/issue-470-anhaenge-bild-pdf.md; no new fragment (no user-visible change).

Acceptance Criteria

• None of the listed types/functions and no MessageContent::Attachments remain (git grep result in the PR).
• A database that applied 0021 and holds an attachments row migrates through 0023 and opens; the row is gone, other messages of the session load (test).
• Fresh database: migrations 0001–0023 run; 0021/0022 byte-identical (checksum test).
• Community behaviour unchanged (existing unit and e2e suites pass).
• Spec 0110, 0034, 0040 and ADR 0137 updated; the changelog fragment is removed.
• Relevant automated tests pass

Technical Notes

• Security-relevant: crates/core/src/ai/ and migrations/. Removing the old path must not weaken the sealing path; its tests stay.
• Merge order: this issue is released by hand once dependents on the old API have moved to the extension point.

Spec impact: update 0110, update 0034, update 0040

Related Work

• Blocked by AI providers: generic binary content blocks and capability check by media type #506; builds on Chat content extension point: opaque extension part in chat messages, history display and removal callbacks #504, Chat content extension point: seal extension content at send time (limits, redaction, fencing, binary checks) #505
• Chat attachments: core message model, fencing, redaction and an attachment resolver slot #469, feat(core): chat attachments with fencing, redaction and resolver slot #475, ADR 0137, ADR 0141